Insulated electric wires each composed of a core wire and a insulating material insulating the same as obtained by molding of a resin are required to have a smaller diameter so as to keep up with the recent increasing trend toward the reduction in size of such articles as mobile devices. Accordingly, it has become necessary to form a thin insulating material layer around the core wire with a smaller diameter.
Polyolefin resins such as polyethylene [PE] and polypropylene [PP] have been used as resins capable of forming thin-walled layers. However, insulated electric wires manufactured by using the conventional polyolefin resins have a problem in that the wall thickness reduction results in increased transmission losses.
As a result of the advancement of communication technology, insulated electric wires are required to be able to transmit large quantities of information. Therefore, it is required that the transmission loss be reduced to a minimum. A method is known for lowering the dielectric constant and thereby improving the insulating characteristics by insulating under foaming (cf. e.g. Japanese Kokai Publication H08-7672). However, such foamed insulating materials are insufficient in strength and, in addition, when the thickness is increased to overcome that drawback, a problem arises, namely it becomes difficult to reduce the insulated wire diameter.
The use of a polyolefin resin as a insulating material presents a problem from the flame retardancy viewpoint, and a method for overcoming this problem which comprises adding a flame retardant is known in the art (cf. e.g. Japanese Kokai Publication H07-182930). However, it is a problem that the addition of a flame retardant results in deteriorations in electrical characteristics.
A method is known for developing sufficient thermal stability to withstand solder reflow processes which comprises using a general-purpose polyolefin resin and curing the same (cf. e.g. Japanese Kokai Publication H06-168627). However, this requires the use of a very expensive apparatus and therefore has an economic problem.
Fluororesins, in particular fluororesins comprising tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymers [PFAs], are excellent in thermal stability, flame retardancy and electrical characteristics and are used in various fields where their characteristics can be displayed. However, PFAs have a problem in that when moldings having minute details or complicated in shape are to be obtained by injection molding, for instance, the moldings tend to undergo surface roughening.
Known as PFA species with which the problem of surface roughening on the molding surface can be solved is a PFA species reduced in molecular weight and having an adjusted molecular weight distribution (cf. e.g. Japanese Kokai Publication 2002-53620). However, when used in insulating molding, this PFA produces a problem, namely an increase in insulating speed results in worsened thin wall forming ability and in a ready tendency toward cone break.
A PFA species further reduced in molecular weight and thus in melt viscosity as well has been investigated as a PFA species with which the problem of reduced thin wall forming ability on the occasion of insulating molding at increased speeds can be resolved. It is a problem of this PFA species, however, that the reduction in molecular weight results in deteriorations in mechanical characteristics.